The present invention generally relates to semiconductor devices, and in particular to a high electron mobility transistor that uses InAs or a mixed crystal thereof for the channel layer.
The high electron mobility transistor known as HEMT is a semiconductor device that utilizes the two-dimensional electron gas formed at the heterojunction interface between a doped compound semiconductor layer called electron supplying layer and an undoped compound semiconductor layer called channel layer acting as the active layer of the device. In this device, the two-dimensional electron gas is formed within the undoped compound semiconductor layer along the heterojunction interface, and the electrons in the two-dimensional electron gas are transported through the channel layer without impurity scattering by the dopant atoms. Thereby, the operational speed of the device is maximized.
Conventionally, group III-V compound semiconductors such as GaAs have been used for the channel layer of HEMT because of the large electron mobility in these compound semiconductor materials. On the other hand, efforts are made in search of materials that provide still higher electron mobility and a larger electron density in the two-dimensional electron gas when combined with a suitable electron supplying layer. Thus, the search of materials for the channel layer of HEMT includes also the search of materials for the electron supplying layer.
So far, InAs and InSb are known as the material that provides the highest electron mobility. For example, InAs shows the effective mass of electrons of only 0.022 m.sub.o in the F-valley of the conduction band, wherein m.sub.o represents the mass of electron. This value is substantially smaller than the effective mass of electrons in the .GAMMA.-valley of GaAs, which is about 0.068 m.sub.o. Similarly, the effective mass of electrons in the .GAMMA.-valley of InSb is only 0.014m.sub.o, which is even smaller than the case of InAs. Further, these materials form a large discontinuity in the band structure when contacted with other compound semiconductor material. This feature is particularly advantageous in forming a large electron density in the two-dimensional electron gas formed at the heterojunction interface of HEMT.
InAs or InSb provides an additional advantageous feature of large transition energy between the .GAMMA.-valley and the X- or L-valley in the conduction band. The latter feature is particularly advantageous for eliminating the real space transfer wherein the electrons, accelerated by the high electric field across the source and gate, cause a transition within the conduction band of the channel layer to the higher energy state corresponding to the X- or L-valley. The electron thus excited subsequently cause a transfer from the channel layer to the lower energy state in the electron supplying layer, moving across the heterojunction interface. The large transition energy within the conduction band effectively suppresses the problem of this real space transfer. Thus, the HEMT that uses InAs or InSb for the active layer is expected to exhibit a superior operational characteristic as compared to the conventional HEMTs.
On the other hand, InAs or InSb has a large lattice constant that is substantially larger than the lattice constant of GaAs or other conventionally used compound semiconductor materials. For example, InAs has a lattice constant of 6.058 .ANG. while InSb has a lattice constant of 6.479 .ANG.A. Thereby, a problem arises about the material that can be used for the electron supplying layer that forms the heteroepitaxial interface with InAs or InSb. Because of these problems, there are limited number of reports about the HEMT that uses the channel layer of InAs or a mixed crystal thereof.
FIG. 1 shows the typical structure of a HEMT, wherein the device comprises a semi-insulating substrate 11 of GaAs and a buffer layer 12 of undoped AlGaSb grown epitaxially thereon.
On the buffer layer 12, a channel layer 13 of InAs is grown epitaxially, and an n-type electron supplying layer 14 of AlSb or GaSb is grown on the channel layer 13. Thereby, a heterojunction interface is formed at a boundary 13a between the channel layer 13 and the electron supplying layer 14, and a two-dimensional electron gas 2DEG is formed along the interface 13a as is well known. On the electron supplying layer 14, there is provided an n-type GaAs or GaSb cap layer 15, and an electrode structure comprising a source electrode 16a of a gold-tin alloy (AuSn), a drain electrode 16b also of the gold-tin alloy, and a gate electrode 16c of aluminum, is provided on the cap layer 15.
At present, GaSb and AlSb are the only conceivable candidates for the electron supplying layer in view point of the lattice matching with the InAs channel layer 13. However, AlSb, containing aluminum, is susceptible to oxidation and causes difficulty in fabrication as well as use of the device. More specifically, AlSb tends to form aluminum oxide during the fabrication process or during the use of the device, and it is necessary to provide a protection to prevent the oxidation. However, such a process complicates the fabrication of the device and hence the reliability as well as the cost of the device.
GaSb, on the other hand, forms the so-called type three heterojunction when formed adjacent to InAs. In the second type heterojunction, there is formed a two-dimensional hole gas in the electron supplying layer along the heterojunction in addition to the two-dimensional electron gas in the channel layer. When such a two-dimensional hole gas is formed simultaneously to the two-dimensional electron gas along the same channel, the controlled flow of the carriers by the electric field applied to the gate electrode 16c does not hold anymore. In other words, the device does not operate properly.
FIG. 2 shows the band structure of the heterojunction interface formed between the InAs channel layer and the GaSb electron supplying layer in the structure of HEMT of FIG. 1. In relation to the small effective mass of the electrons and holes, InAs or the mixed crystal thereof generally has a relatively small band gap between the valence band Ev and the conduction band Ec. Particularly, when combined with GaSb electron supplying layer, the edge of the conduction band Ec of the InAs channel layer 13 is located at a level lower than the edge of the valence band Ev of the GaSb electron supplying layer 14. In this system, it should be noted that the Fermi level E.sub.F is located between the conduction band of the InAs channel layer 13 and the valence band of the GaSb electron supplying layer 14 as illustrated. Thus, there is formed the two-dimensional hole gas designated as 2DHG adjacent to the two-dimensional electron gas designated as 2DEG. As the holes behave oppositely to the electrons under the electric field, the simultaneous existence of 2DEG and 2DHG causes a serious problem in the operation of the HEMT as already described.